Use message and exception listeners in .NET

A .NET application uses a message listener to receive messages asynchronously, and it uses an exception listener to be notified asynchronously of a problem with a connection.

The functionality of both the message and exception listeners is the same for .NET and for C++. However, there are some small implementation differences.

Procedure

To set up a message listener in order to receive messages asynchronously, complete the following steps:
1. Define a method that matches the signature of the message listener delegate. The method that you define can be either a static or an instance method and can be defined in any accessible class. The delegate signature is as follows:
```
public delegate void MessageListener(IMessage msg); 
```
  and so you could define the method as:
```
void SomeMethodName(IMessage msg);
```
2. Instantiate this method as a delegate using something similar to the following example:
```
MessageListener OnMsgMethod = new MessageListener(SomeMethodName)
```
3. Register the delegate with one or more consumers by setting it to the MessageListener property of the consumer:
```
consumer.MessageListener = OnMsgMethod;
```
  We can remove the delegate by setting the MessageListener back to null:
```
consumer.MessageListener = null;
```
To set up an exception listener, complete the following steps. The exception listener works in much the same way as the message listener, but has a different delegate definition and is assigned to the connection rather then the message consumer. This is the same as for C++.
1. Define the method. The delegate signature is as follows:
```
public delegate void ExceptionListener(Exception ex);  
```
  and so the method defined could be:
```
void SomeMethodName(Exception ex);
```
2. Instantiate this method as a delegate using something similar to the following example:
```
ExceptionListener OnExMethod = new ExceptionListener(SomeMethodName)
```
3. Register the delegate with the connection by setting its ExceptionListener property:
```
connection.ExceptionListener = OnExMethod ;  
```
  We can remove the delegate by resetting the ExceptionListener to:
```
null:  connection.ExceptionListener = null;
```

Parent topic: Writing XMS .NET applications